Developers of information storage devices and methods continue to seek increased storage capacity. As parts of this development, so-called page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices.
In the typical holographic storage system, two coherent light beams are directed onto a storage medium. The first coherent light beam is a signal beam, which is used to encode data. The second coherent light beam is a reference light beam. The two coherent light beams intersect within the storage medium to produce an interference pattern.
The recorded information, stored as a holographic image, can be read by illuminating the holographic image with a reference beam. When the holographic image is illuminated with a reference beam at an appropriate angle, a signal beam containing the information stored is produced. Most often the appropriate angle for illuminating the holographic image will be the same as the angle of the reference beam used for recording the holographic image. More than one holographic image may be stored in the same volume by, for example, varying the angle of the reference beam during recording.
A hologram may be recorded in a medium as a variation of absorption or phase or both. A holographic recording material must respond to incident light pattern by causing a change in its optical properties. In the absorption or amplitude modulating materials, the absorption constant (or extinction coefficient) of the media changes as a result of exposure of the incident light, which is significantly absorbed in the medium. On the other hand, in phase modulating materials, the thickness or the refractive index changes due to the exposure. In the phase modulating materials there is generally very little absorption of light and the entire incident light is available for image formation. Thus, a phase modulating material can produce a higher efficiency than an amplitude modulating material. Also, in phase modulating media the amount of phase modulation could be made substantially large by tuning the refractive index changes in the material.
In early versions of holographic media, e.g., silver halide media, a latent image was first recorded in a silver halide emulsion. The latent images were then developed and fixed. During the development process the exposed silver halides in the emulsion are chemically reduced to metallic silver. The unexposed silver halide crystals remained in emulsion after development. These were still photosensitive and limited the life of the developed emulsion. They were removed by “fixing” with sodium thiosulphate (hypo), which formed a number of water-soluble silvery complexes along with a few water-insoluble complexes
Such processes of the silver halide media involved use of chemicals and thin holographic media. Silver halide materials were the most popular choice of the early holographers for obvious reasons of high exposure sensitivity over a wide range of spectral wavelengths and high resolving power. These materials were suitable for transmission as well as reflection holograms, both of amplitude and phase type. A large number of developers, bleaches and processes have been reported for silver halide materials. However, silver halide materials were found to lack high dynamic range (ability to store data), had poor archival stability, and were difficult to work with because of the chemical processing needed to develop and fix the holograms.
More recently, a polymeric holographic medium has been the material of choice. A polymeric holographic medium records the interference pattern by changing its index of refraction to form an image of the interference pattern. Such photopolymers are capable of producing large index modulation and high diffraction efficiencies. Photopolymers do not require lengthy controlled processing techniques and can be naturally self developing (via diffusion processes) in situ and the fixation step could be accomplished by exposure to incoherent light. The photopolymer holograms are insensitive to environmental changes.
Photopolymers for holographic media represented a breakthrough, since media could then be framed between two hard substrates and also be made to any thickness. The result was holographic media with high dynamic range, good sensitivity, excellent shelf and archival life, and ease of manufacturing. Media such as this is described in U.S. Pat. No. 6,482,551 (incorporated herein by reference). This class of media is ideally suited for high density holographic data storage, and much progress has been made in the field of holographic data storage using photopolymeric media.
However, the ability of the photopolymer class of holographic media to self develop does result in a problem when large numbers of holograms are recorded into the same volume (which is required for high density data storage). Each recorded hologram in the photopolymer matrix is a spatial refractive index change in the media. Thus, as more and more holograms are written to the same volume of space, a very complex pattern of spatial refractive index is created within the photopolymer matrix. Ideally, the recording of later holograms in this same volume should not interact with the previously recorded holograms (this is generally true for a small number of multiplexed holograms). In practice though, as more holograms are recorded into the same volume, the recording of later holograms is affected by the existing holograms. With each additional recorded hologram, the media becomes more spatially inhomogeneous with respect to light transmission. This spatial inhomogeneity (or modulation) causes the later writing beams to diffract, creating undesired optical noise that in turn writes undesired modulation components into the photosensitive medium, e.g., the photopolymer matrix. This recording of secondary diffracted light can be a significant cause of signal degradation, and therefore, diminished storage density. Furthermore, light diffracted from the secondary modulation will record tertiary modulation and so on, so that many orders of rediffracted noise light are created and recorded.
In short, the capabilities of holographic storage systems are limited in part by the storage media. No single material possessed all the requirements of a holographic material. Thus, there is a need for material that would have the high sensitivity and latent image of silver halides while at the same time having the high diffraction efficiency and index modulation capabilities of photopolymers.